The sense of balance utilizes input from the vestibular end organs, indicating head movements and positions. While thousands of afferent nerve fibers transmit the information from the periphery to the central nervous system,. Only a few hundred, the efferent vestibular neurons, are used to control and modify the end organ function with commands originated in the brain. The understanding of the function of the efferent neurons is essential for the understanding of the function of the vestibular system in health and disease. This small grant will be used to gather preliminary data on the function and physiological characteristics of the efferent vestibular neurons in the chinchilla. Recent work on efferent-mediated responses recorded from vestibular nerve-afferents suggested that the efferent vestibular system (EVS) conveys vestibular input to modify subsequent input from the vestibular end organs. Yet, the structural organization of the EVS pathways, as well as the role of the efferent neurons in the process are not clear. The specific aims are: 1) To study the physiological properties of EVNs in the decerebrate chinchillas by recording EVN responses to vestibular stimuli. In doing so we shall test the hypothesis that the EVNs use primary vestibular input to modulate subsequent input from the vestibular end organs. The organizational structure of efferent related pathways will be determined. Primary vestibular influence on the efferents will be dissected by responses to head rotations and tilts, to electrical stimulation (shocks and current steps) of the ipsi- and contralateral labyrinths and by the use of preparations with selective canal plugging. 2) To examine the hypothesis that the EVS is part of an unstable unanesthetized decerebrate chinchillas, of large fluctuations in afferent discharge rate, which we have ascribed to the positive feedback loop between afferents and the isilateral EVNs. A non-linear model developed to explain the dynamics of the feedback loop suggests, for example, that if loop gains are high, the system will be driven into large fluctuations in firing rates of its elements, similar to these that have already been seen while using direct electrical stimulation of efferent neurons on the ipsilateral side. By doing this we control the activation parameters of the EVS, in particular the ipsilateral feedback loop. The significance of this research is in setting the foundations of mammalian efferent neuron physiology, and providing experimental data on the control mechanism by which the EVS operates.